The invention relates to a volume or mass flowmeter with a flow sensor and with evaluation electronics.
For the purpose of measuring the flow rate of a fluid flowing in a pipeline or the like there are several principles which are each based on a physical regularity. Irrespective of the electric conductivity of the fluid, its volume flow rate can be measured, for example, by vortex flowmeters based on the Kxc3xa1rmxc3xa1n vortex street or by ultrasonic flowmeters or its mass flow rate can be measured, for example, by mass flowmeters which are based on the Coriolis principle, by thermal mass flowmeters or by mass flowmeters which are based on the determination of a pressure difference over an orifice plate. The volume flow rate of electrically conductive fluids can further be measured with electromagnetic flowmeters based on Faraday""s law of induction.
The evaluation electronics convert a signal as generated according to one of the aforementioned principles into an output signal proportional to the volume or mass flow rate with high measuring accuracy. For example, this can be the DC current with 4 mA to 20 mA for a given measuring range which has long been used in industrial metrology. However, the invention does not deal with this kind of output signal.
Rather, the invention is aimed at the elimination of a disadvantage which can occur in an alternating output signal, which has also been standardized for a long time in industrial metrology and whose frequency is proportional to the flow rate in the given measuring range, whenever the fluid flowing in a measurement direction due to the mechanical arrangement of the flow sensor flows occasionally, and for a short time in particular, in the opposite direction of the measurement. Such a counterflow rate cannot be converted by the evaluation electronics into a respective counterfrequency because, as is well known, an alternating signal cannot have a negative frequency.
The output at which the standardized output signal aforementioned at last lies is usually designated in technical manuals and data sheets as pulse/frequency output.
The aforementioned counterflow rate occurs, for example, when the flow rate is to be measured in a pipeline in which the fluid is not moved continuously but instead in a pulsating manner, e.g. by metering pumps or, in the case of fluids placed under pressure, by valves which are triggered to be open or closed. Although metering pumps have a very high precision of up to 0.5%, return flows, which also includes counterflow rates, occurring during each metering step cannot be avoided as a result of their principle of design (e.g. reciprocating pump, bellow-type pump or diaphragm pump).
Efforts were made up until now to compensate counterflow rates by damping in the evaluation electronics. As a result, the flow rate measurement only reacts with delays to changes in the flow rate and in addition it is necessary that the time constant of the damping has to be adjusted to the flow rate, i.e. the time constant must be changed and must be changeable.
It is an object of the present invention to provide a different and more advantageous approach for the elimination of the aforementioned disadvantage.
In order to achieve this object, the invention therefore consists in a volume or mass flow meter with a flow sensor and with evaluation electronics, comprising:
a first subcircuit for the generation of a flow rate signal,
which in the given measuring range is proportional to the flow rate of the fluid to be measured,
a second subcircuit for generating an output signal,
whose frequency in the given measuring range is proportional to the flow rate of the fluid flowing in a measuring direction determined by the constructional arrangement of the flow sensor, and
a third subcircuit,
which determines a counterflow rate in an opposite measurement direction during a scanning interval, saves the same and subtracts it from the next measured flow rate.
In accordance with a preferred development of the invention the evaluation electronics add up the saved counterflow rates separately and make them available for further processing and/or display.
In accordance with a preferred embodiment of the invention and/or its preferred development, the third subcircuit comprises:
a clock generator,
which periodically generates clock pulses with a predetermined clock period,
an averager stage controlled by the clock generator,
whose input is supplied with the flow rate signal and whose ouptut supplies an average flow-rate value signal representative of an average taken over the clock period,
a write-read memory,
a first divider,
of which a divisor input is fed with a clock period signal representative of the clock period and
of which a dividend input is connected to an output of the write-read memory,
a first summer,
of which a first input is connected to an output of the averager stage and
of which a second input is connected to an output of the first divider,
a first multiplier,
of which a first input is connected to an output of the first summer,
of which a second input is supplied with a setting signal representative of a reciprocal mass or a reciprocal volume and
of which an output supplies a frequency signal representative of a frequency,
a triple comparator,
of which an input is connected to output of the first multiplier and
of which an output is connected to an input of the second subcircuit and supplies a signal which
represents zero when the frequency signal represents values smaller than zero,
is equal to the frequency signal when the same represents values between zero and an adjustable maximum value and
represents the maximum value when the freqeuncy signal represents values larger than the maximum value,
a second multiplier,
of which a first input is connected to the output of the triple comparator and
of which a second input is supplied with the clock period signal,
a second divider,
of which a first input is connected to an output of the second multiplier and
of which a second input is supplied with the setting signal,
a third multiplier,
of which a first input is connected to the output of the averager stage and
of which a second input is supplied with the clock period signal,
a summer/subtracter of which
a subtrahend input is connected to the output of the second divider,
a first addend input is connected to an output of the third multiplier,
a second addend input is connected to the output of the write-read memory and
an output is connected to an input of the write-read memory which is enabled by the clock signal.
One advantage of the invention is that a counterflow rate occurring at a specific time will already be considered by balancing during the measurement of the next flow rate value, i.e. in real time. Accordingly, in accordance with the invention the balancing follows a change in the flow rate without any delay, so that it is also not necessary to change any time constants.